1. Field of the Invention
This invention relates to a method to enhance the elevated temperature performance of tin coated electrical and electronic articles. More particularly, a zinc layer is deposited on the tin coating thereby reducing the contact resistance of articles exposed to elevated temperatures for extended periods of times, for example, 150.degree. C. for seven days or for 125.degree. C. for 1000 hours.
2. Background of the Invention
Copper and copper alloy substrates are formed into articles for use as electrical and electronic components, such as electrical connectors and leadframes. Copper and copper alloys readily oxidize when exposed to oxygen containing atmospheres and readily tarnish when exposed to sulfur containing atmospheres. Both oxidation and tarnish are exacerbated at elevated temperatures, defined herein as temperatures above about 125.degree. C. Since air contains oxygen as a major constituent and sulfur as a common pollutant, under the hood automotive connectors and appliance connectors are exposed to oxidizing and tarnishing environments.
The copper or copper alloy substrate may be coated with a layer of tin to inhibit surfaces of the copper or copper alloy article from oxidizing or tarnishing. A tarnish-free and oxide-free surface has lower electrical contact resistance and better solderability than an oxidized or tarnished surface.
When exposed to an oxidizing atmosphere at elevated temperatures, tin coatings are prone to oxidation. The oxide film is typically only about 50-200 Angstroms in thickness, but the surface oxide imparts the article with a yellow color that many consumers consider unacceptable. If sufficiently thick, the oxide layer may increase the contact resistance of the tin coated article.
A publication entitled, "An Examination of Oxide Films on Tin and Tin Plate", by S. C. Britton and K. Bright discloses that alloying small amounts, on the order of 0.1%, of phosphorous, indium or zinc with tin either prevents or reduces the formation of tin oxide when heated to 210.degree. C. for 18 hours.
Japanese Kokai No. 3 (1991)-239,353 published Oct. 24, 1991, discloses a copper leadframe for semiconductor devices having a zinc layer disposed between the copper substrate and a tin-base tin/lead solder coat. The zinc layer is disclosed to be a barrier layer that reduces interdiffusion between the tin and the copper leading to enhanced solder wettability when heated.
Other barrier layers disposed between a copper alloy substrate and a tin coating layer are disclosed in U.S. Pat. No. 5,780,172 by Fister et al. that is incorporated by reference in its entirety herein. The patent discloses copper/nickel barrier layers in the form of both a copper base alloy as multiple layers. The inclusion of a zinc layer in the barrier is also disclosed.
An article, such as a copper or copper-base alloy electrical connector or electronic component, may coated with a tin or tin-base coating (The term "base" is intended to convey that the alloy contains at least 50%, by weight, of the specified element. Tin or tin base coatings will be referred to herein as tin coatings. All percentages are in weight percent unless otherwise specified.) by any one of a number of conventional processes such as electroplating, hot dipping, electroless chemical deposition, vapor deposition or cladding.
Electroplating electrolytically deposits tin from a tin ion containing electrolyte on to a cathodically charged article. Examples of such baths include tin fluoborate, tin methane-sulfonic acid, tin sulfate and stannate. One exemplary electrolyte contains between 10 g/l and 50 g/l of tin and between 30 g/l and 70 g/l of sulfuric acid. This bath is typically acidic and operated at a nominal temperature of 20-40.degree. C. at a current density of about 30 amps per square foot. The bath will deposit about 50 microinches of tin in 1 minute.
The tin coating layer may be bright or matte dependent on the electroplating conditions. A bright finish may be achieved by adding an organic material, for example, polyethylene glycol, to the tin bath. The addition of such an organic material produces a tin coating with a smooth, hard surface and high reflectivity.
Reflectivity may be evaluated by the "ruler test". A conventional ruler is extended vertically from a horizontally lying sample. The highest number that can be clearly discerned in the tin plate reflection is deemed the reflectivity.
A matte finish is a semi-bright, satin, finish that is typically thicker than the bright finish. While cosmetically less appealing, matte coatings tend to have a longer service life and are typically used in heavy-duty applications.
The tin coating may also be deposited by a HALT (hot air level tin) process. The article is immersed in a molten tin bath and wet by the tin. When the article is withdrawn from the molten tin bath, jets of high velocity hot air are directed across major surfaces of the article. The hot air levels the tin coating at a desired thickness such as between 40 to 400 microinches thick.
In place of the HALT process, a mechanical wipe process may be. An article is immersed in a molten tin bath and, upon withdrawal from the bath, the article is physically wiped such as with steel rods, glass rods or wire brushes. The thickness of the tin layer is a function of how much tin is wiped from the surface. The thickness of the tin coating is typically between about 20-80 microinches, with a preferred thickness of approximately 25-50 microinches.
Notwithstanding the method of tin deposition, most electrical and electronic articles are coated with between 15 and 200 microinches of tin.
A tin-lead solder coating may also be deposited on the article by any of the above methods. Typical solders have from 5% to 95% tin and the balance is lead. Preferably, the solder coat is from 25% to 75% tin and the remainder lead. Two common solders are 60%/40% Sn/Pb and 63%/37% Sn/Pb.
There remains, however, a need for a method to impart tin coated articles that will be exposed to oxidizing and tarnishing atmospheres at elevated temperatures with enhanced solderability and reflectivity while reducing an increase in contact resistance following exposure to elevated temperatures.